This invention relates to optical radars in which a high power CO.sub.2 laser is used as a transmitter and in which target enchoes are heterodyned with the output of a local oscillator laser to yield an intermediate frequency signal in the RF region, for example in the VHF or UHF band. The desired target information is then extracted from the intermediate frequency signal.
CO.sub.2 lasers are preferred as the transmitters of optical radars because of the high electrical efficiency and high power characteristics thereof, because the emitted radiation thereof is in the infrared region at approximately 10 microns wavelength and is thus both convert and eye safe, and also because the atmospheric low-loss transmission window which exists between 8 and 14 microns makes possible long range optical transmission. High powered CO.sub.2 transmitter lasers necessarily involve moderate to large Fresnel number optical cavities which have inherently unacceptable temporal and modal stability. The temporal instabilities arise when the differential optical loss among competing high order transverse and longitudinal modes is low, hence the laser oscillator indiscriminately "mode hops". Moreover, without some form of intracavity optical dispersion, a high gain CO.sub.2 laser transmitter can oscillate on any number of vibrational-rotational transitions in the 9 to 11 micron spectral region, and while gratings or prisms may be employed to provide intracavity optical dispersion, these elements invariably and considerable optical loss.
These inherently unstable large CO.sub.2 lasers can be stabilized or controlled by injecting into the cavity thereof a small sample of the desired frequency, wavelength and mode of operation, as long as the high power laser cavity has the required optical design to support this frequency or wavelength of oscillation. Under these conditions, the injected signal will force the higher powered device to operate on the injected transition and transverse mode. The source of the desired injection signal is usually another smaller CO.sub.2 laser which, due to its smaller cavity dimensions, has much better temporal, mode and frequency stability, and which can in addition be provided with an accurate frequency stabilization system, which may include, for example, a Stark cell as an absolute frequency reference.
Heterodyne optical radars require highly stable transmitters and local oscillators. If the desired radar signature is of the Doppler type, any frequency drift between the transmitter and local oscillator will have the same effect in the intermediate frequency (IF) signal thereof as radial target movement. The prior art includes homodyne type optical radars in which a frequency stable local oscillator laser has had a portion of its output injected into the high power transmitter laser so that both lasers operate at the same frequency. Such a homodyne radar cannot distinguish the sense of radial movement of moving targets since it in effect has a zero intermediate frequency. Further, homodyne radars have the additional disadvantage that they do not produce any video signal for stationary targets and they produce only extremely low frequency video signals for targets with slow radial motion, and this limits the detection of low speed radially moving targets.
Some of these disadvantages can be overcome by injecting the output of a single local oscillator laser into the cavity of the larger CO.sub.2 transmitter and selecting an axial mode therein which has a frequency different from the injected frequency. This results in a heterodyne radar with a non-zero IF which can distinguish the sense of target radial movement, but the selection or choice of the intermediate frequency is constrained by the available axial modes of the transmitter, and further it may require operation of the transmitter laser off of its line center where the output beam power is not a maximum.
In contrast with these prior art optical radars the present invention provides a more versatile heterodyne radar in which the transmitter is injection-controlled so that it operates at a highly stable frequency which is offset in frequency by a fixed and controllable amount from the local oscillator laser. The amount of frequency offset determines the intermediate frequency.